Justin asked:

Why does mortality of the soul follow from it being the form of the substance according to Aristotle?

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The Aristotelian soul is mortal because no part is separable from body. At De AnimaBook 3 Chapter
5 Aristotle distinguishes a passive and an active intellect and the latter is described as "separate,
unaffected and unmixed, being in substance activity" and "it is this alone that is immortal and eternal".
However, this is a suggestion that doesn't cohere with the main account he gives of the soul as a
hierarchical set of faculties, the higher dependent on the lower, nor with his description of the soul as
"homoeomerous", which to say that it is not divisible into parts, except conceptually. The soul is the
life force of a living organism, or what it is to be alive, and when the physical matter of an individual is
destroyed so too is the soul or life force. A further reason why the soul is mortal is that form and
matter are inseparable because the form is the identity and definition of a being.

For Aristotle, the soul as form is physiological. Sensation is not clearly distinguished from perceiving,
but both are part of complex process which is a "receiving of form without matter" by a sense organ
and this is a non-propositional state shared with animals. Thought is dependent on this physiological
process because our capacity to form images is dependent on perception, or as Aristotle says,
imagination is "an affection belonging to the common sense". The abstract thinking of the active
intellect implies no physiological process, but there is a dependency if Aristotle's claim that there is no
thinking without images is accepted.

Rachel Browne